The present invention relates in general to polymeric resins and the handling of the dry polymer product. The invention relates in particular to polyvinyl chloride (PVC) resins, their preparation, and their handling properties. The invention also relates to a polymerizable reaction mixture for producing high bulk density PVC resins and the process for their preparation.
It is highly desirable to be able to consistently prepare high bulk density homopolymer and copolymer PVC resins; i.e., resins prepared from predominantly vinyl chloride monomer (VCM). This has not been achieved in the past because of the inability to consistently polymerize the VCM reaction monomer mixture so as to produce a consistently high bulk density product having other acceptable properties. Various published procedures have not been found satisfactory for consistently producing high bulk density PVC resins with good handling and other physical properties. It is desirable that the PVC resins have good heat stability with the occurrence of few fish-eyes, good particle size distribution, good plasticizer absorption (porosity), and with a structure that easily permits essentially complete removal of the residual VCM.
In conventional PVC suspension polymerization, vinyl chloride and optionally other ethylenically unsaturated monomers are suspended in water by vigorous stirring and suspension agents. Small droplets are created and, with the help of catalysts (usually peroxide initiators), are converted to solid polymer particles during polymerization. These suspension agents are typically water-soluble protective colloids which maintain the droplets as separate entities and prevent partly polymerized monomer droplets from agglomerating together. The polymerized particles deposit as a powdery product with an apparent density, pour weight, or more typically, bulk density. The bulk density is a very important factor since a higher bulk density for the PVC product permits better space utilization for processing containers, mixers, loaders, storage silos, rail cars, processing machines such as extruders and plasticizers and other containers. The output of the processing machines is increased for higher bulk density PVC product. An especially important aspect is the improved output rate on extrusion for high bulk density suspension PVC resin.
The PVC resins of homopolymer and copolymer materials are widely used in industry for production of pipe, siding and many other products. Recently, the PVC resin products have been used in markets previously utilizing only steel, iron, and asbestos-cement materials. For vinyl siding, the PVC resins have improved cost effectiveness, impact resistance, and weatherability over conventional siding and have replaced wood and aluminum for such purposes.
Despite the recognized need for a consistently high bulk density PVC resin, there has not been produced a consistently high bulk density PVC resin having the other requisite properties to assure good processing characteristics and production of finished products of high quality. The resin must possess a relatively narrow particle size distribution as well as good porosity. Although some high bulk density PVC resins have been produced, their porosity has generally been so low and inconsistent that demonomerization (removal of residual VCM) of the resins was extremely difficult and it was impossible to consistently guarantee removal of the monomer to the extent required by governmental regulations. The skilled artisan readily recognizes that the removal of VCM is critical since it has been determined to be a liver carcinogen. In addition, high bulk density PVC resins prior to the invention have generally had a wide particle size distribution. Other processes for the production of high bulk density resin have had somewhat better results but are formed by a process so complicated as to be commercially infeasible.
The PVC resins of the invention are prepared by a suspension polymerization process and have not only a consistently high bulk density but a good balance of porosity and particle size distribution as well.